Microphone



L. DE FOREST Dec; 1, 1931.

MICROPHONE Filed Oct. 2, 1929 awuwntoz 1 ea De Ff'eof' @351 5 6 Game/1s.

Patented Dec. 1, 1931 UNITED STATES PAETENT OFFICE LEE DE FOREST, OF NEWYORK, N. Y ASSIGNOR TO GENERAL TALKING PICTURES COR- PORATION, OF NEWYORK, N. Y.,

A CORPORATION OF DELAWARE mcnoPHoNE Application filed October 2, 1929;Serial No. 396,642.

This invention relates in general to microphone pick-up devices fortranslating sound waves into varying electrical currents.

One of the objects of the invention is to obtain a microphonedevicewhich is at once small, free from carbon noises, sensitive and notsubjectto the difliculties normally encountered in securing andmaintaining perfect insulation between electrodes as is fre- 1o quentlythe case with microphones of the carbon or electrostatic types. Anotherobject of this invention involves a microphone having an incandescentcathode and a movable anode responsive to sound to waves.

Another object of this invention is to provide a microphone of the abovetype in which the cathode operates either in a vacuum or a gaseousatmosphere either above or below atmospheric pressure.

A still further object of this invention is to provide a microphonewhich possesses a lower impedance than impedance of an electrostaticmicrophone.

Other objects of this invention involve features of construction, gasesemployed as well as pressures, and other characteristics,

i all as will be described in greater detail below. 9

This invention resides substantially in the combination, construction,arrangement and relative location of parts all as will be described infurther detail below.

Referring to the drawings 0 Figure 1 is a longitudinal cross-sectionalview through one form of microphone employing the principles of thisinvention;

Fig. 2 is an end elevational view of this device; a

Fig. 3 is a plan view of a modified form of cathode;

Fig. 4 is a side elevational view of the modified cathode with someparts. in cross-section; 7

Fig. 5 is another form of microphone operating at greater thanatmospheric pressure;

Fig. 6 illustrates still another form operating below atmosphericpressure;

Fig. 7 illustrates still another form of the device operating belowatmospheric pressure with a modified form of anode;

Fig. 8 is a cross-sectional view taken on the line 8-8 of Fig. 7.

As is well known the many forms of microphones now in use have variousundesirable characteristics peculiar to each type. For example, thecarbon microphone and the electrostatic microphone are sources ofundesirable noises which reduce their efficiency of operation; theelectrostatic t pe whose undesirable noises are in part properinsulation in their construction, and in the carbon microphone due tothe presence of the high resistance carbon contacts. The microphone ofthis invention possesses a lower impedance than those generally usedand, therefore, permits the use of a long cable be ue to im-' tween itand the device to which it is connected, which is not true of amicrophone of electrostatic type, for instance, it does not operate wellwith a long cable connection such as is normally required, for example,in making talking motion pictures. These known .forms of microphones,particularly the electrostatic types, are seriously afi'ected bymoisture conditions. The device of this invention is not so affected.The microphone on this invention is also particularly adapted for smallconstructions which is highly desirable in some uses.

For a better understandin of the inven tion and its advantages re erencewill be made to the drawings. In general the microphone of thisinvention may be said to be of the two-electrode vacuum tube type. Theform of a device shown in Figure 1 comprises a metal c linder 1 havinga. chamzfered edge to which is welded the lass member 2 at the jointindicated at 2 to orm a hermetic seal. In accordance with resent weldingpractice the tube 1 is pre erably of copper. Byreducing the edge of thetube to make it thin the glass portion may be welded thereto in a mannernow well known in the welding art. The glass portion 3 is provided witha re-entrant stem or tube 4.- which is pressed to gether at the point 5while hot to hermetically seal the tube at the point where the wiresenter the vessel. The glass portion of the container may be evacuatedand sealed as at the points 6. The end of the tube 1 is closed by themetal cap 7 which is of thin material and forms a hermetic seal with thetube 1. Supported on stiff wires 9 is a cathode 8 adapted to be heatedto incandescence and preferably coated with an electronically activeoxide coating. The wires 9 are connected to the supporting wires 10which are sealed in the brace 5 and to which are connected the leadwires 12. The cap 7 forms the anode of the device and may be made ofthin copper, nickel, tantalum, duraluminum, or other suitable materialpressed in the form of a cap. The wires 12 are connected across thevariable resistance 13 to the current source 14 which is used toenergize the cathode 8. The casing 1 is provided with a binding post 18which is connected by a wire to one terminal of the primary 17. of asuitable transformer. The other terminal of the primary is connectedthrough the current source 16 by wire 15 to one of the wires 12.. Thesecondary 19 of the transformer is connected by the wires 20 to anydevice to be actuated by the currents produced by the microphone, suchas the input of an amplifier. The tube 4 is shown by this form of thedevice as extending beyond the brace 5 to help to support the cathode 8and its supporting wires 9. The complete vessel formed by the tube 1,glass member 3 and cap 7 may be exhausted preferably to a high degreeof-va'cuum and sealed off. The device will, of course, operate whenexhausted even to a low degree. In certain cases it is pref.- erable toemploy some gas pressure other than air such as that.provided byplacinga globule of mercury within the container and exhausting it to thepressure of mercury vapor. In addition to or in lieu of the mercury oneof the noble gases such as argon,

krypton and zenon, either pure or in any desired mixture thereof may beemployed. Although the mercury tends in time to become amalgamated withthe copper tube the device still retains the qualities of amercury'vapor device. When the pressure within the device isbelowatmospheric pressure the thin diaphragm or anode 7 tends to bendinwardly into a slightly concave form and puts the diaphragm underconsiderable tension. For good results the diaphragm is initiallysecured to the tube 1 under tension which tension is enhanced by theatmospheric pressurethereon. This tension in the diaphragm materiallyreduces the natural period of vibration. In one desirable form theperiod of vibration of the diaphragm is well above one six thousandthsof a second. In preferable form the cathode 8 is placed close be hindthe anode 7 at a distance of the order of one-sixteenth of an inch.

When sound waves are directed against the dlaphragm they impart theirvibrations thereto to alternately move the diaphragm closer to andfurther from the cathode member thereby periodically altering theimpedance of the device by andin accordance with the impinging soundwaves. This action results in corresponding linear changes in theelectronic or ionic currents passing between the cathode and ,theanode.These electronic shown at 8 in Figs. 3 and4. To prevent sagging of thespiral it may be supported on the disc 21 of insulating material such asmica. The mica disc may be supported from the brace 5 by means of thesupport wires 9 and additional support wire such as the wire 22. It is,of course, apparent that the cathode may be made in many forms withoutdeparting from this invention.

Animportant advantage of this construction of microphone is that itmaybe made extremely small. Thus its overall dimensions may be of theorder of one inch across and two to three inches in length. Such a smallmicrophone maybe readily concealed and-is hence very well adapted foruse in the making of talking motion pictures since it may be easilylocated close to the person speaking without it being observable in thepicture when thrown on the screen.

In the forms of the device shown in Figs. 5 and 6 the diaphragm 7 ismade of rubber or rubberized fabric instead of metal and hermeticallysealed to the tube 1. The form shown in Fig. 5 is intended to operate atgreater than atmospheric pressure and the tube 23 connected thereto tothe valve 24 is employed to supply air or other gases alone or incombination at greater than atmospheric pressure to the device. In thiscase the diaphragm will convex outwardly. In the form of device shown inFig. 6 the device is operated below atmospheric pressure and, as aresult, the diaphragm will convex inwardly. Since a diaphragm of thistype might permit leakage in time the tube 23 which contains valve 24 isprovided to correct the pressure condition within the device from timeto time as it becomes necessary. In the case of a diaphragm of thisnature its inner surface must be metallized to provide a conductingsurface which will, of course, be in contact with the tube 1 to whichone connection is made. Instead of metallizing the inner surface ofthediaphragm the construction shown in Figs. 7 and 8 may be employed. Inthis case the anode comprises a supporting metallic ring 25 for a mesh28. The anode is supported by the wires 26 and 27 from the for the anodeconnection. In the operation of the forms of device shown in Figs. 5 to8 inclusive, the vibration of the flexible diaphragm in accordance withimpinging sound 5 Waves produces alternate compression and a'rarefaction of the gas within the chamber regardless 'of its pressurerelative to the atmosphere. These changes of pressure within the devicecontrol the amount of ionic current passing from. the cathode to theanode and hence the impedance of the device.

It will be apparent from the foregoing disclosure that I have embodiedcertain principles of construction and operation in a new form ofmicrophone device which principles may be embodied ,in other physicalforms without departing from the spirit and scope of this invention. Ido not, therefore, desire to be strictly limited to the form ofdisclosure 20 given for purposes of illustration but rather to thespirit and scope of the appended claims.

What I seek to secure by United States Letters Patent is:

1. A microphone device of the type described, comprising a glass portionand a metal portion hermetically sealed together, a

conducting flexible portion hermetically sealed to the metal portion toform an anode and a cathode within said vessel supported adjacent theflexible portion, said vessel being exhausted.

2. A microphone device of the type described, comprising a glass portionand a metal portion hermetically sealed together, a

conducting flexible portion hermetically sealed to the metal portion toform an anode and a cathode within said vessel supported adjacent theflexible portion, said vessel con- 40 taining a mixture of gases. I

3. A microphone device of the type described, comprising a metal portionand a glass portion hermetically sealed together,

said glass portion having a reentrant stem, a

' flexible diaphragm hermetically sealed to the metal portion to form aclosed vessel and a cathode supported on said reentrant stem adjacentsaid flexible diaphragm, said flexible diaphragm acting as a soundresponsive anode with respect to said cathode.

4. A microphone device of the type d scribed, comprising a metal portionand a glass portion hermetically sealed together, said glass portionhaving a reentrant stem, 9. flexible diaphragm hermetically sealed tothe metal portion to' form a closed vessel, a

cathode supported on said reentrant stem ad-" jacent said flexiblediaphragm, said flexible diaphragm acting as a sound responsive anodewith respect to said cathode,-current leads extending through saidreentrant stem 's for said cathode, and; means connected to said leadsfor energizing said cathode.

5. A microphone device of the type described comprising an exhaustedvessel having a portion thereof of flexible non-conducting materialmounted under tension and having its surface exposed to the interior ofthe Vessel metallized, said diaphragm forming an anode, and a hotcathode electrode within said vessel. A

6. In a. microphone device of the type described the combinationcomprising an exhausted vessel having a portion-of its wall made up of arubber diaphragm, said diaphra m being metallized on its surface exposeto the interior of the vessel to provide an anode and a cathode withinsaid vessel mounted adjacent the anode.

In testimony whereof I have hereunto set my hand on this 30th da ofSept., A. D. 1929.

IYEE on FOREST.

